I've narrowed this down to something that happened around May 8-10. I'm
seeing a roughly 10% increase in compile times that was in the May 10
nightly build compared to the May 8 nightly build (I've unpacked the
builds to check).

Re-do the "function application discount" (fixes Trac #6048)
}}}
(the SimplCont patches also made things a bit worse, but I haven't looked
into that at all).

I think that the problem is more/bigger inlinings, rather than a space
leak in GHC itself. I'll attach `CopyFile.hs`, and `before.txt` and
`after.txt` showing the interface files before and after this patch is
applied.

Interface file size:
{{{
GHC 7.4.1 -O -rw-rw-r-- 1 simonpj GHC 5983 Jul 16 15:21 T6104.hi
HEAD -rw-rw-r-- 1 simonpj GHC 10487 Jul 16 14:08 T6104.hi
}}}
So, yes, the interface file is a lot bigger. Looking more closely, this
seems largely accidental. The code in HEAD is broken into more functions,
each of which is individually small enough to be below the inlining-
creation threshold.

I compiled Cabal with -O, and compared the total size (with `ls -l`) of
the `.o` files and `.hi` files with both compilers:
{{{
HEAD change over 7.4.1
.o file size -6%
.hi file size +8%
}}}
So yes, interface file size is up, but not massively, and .o file size is
down.

This does not seem to account for the originally-reported doubling of
residency! And (I've checked) that doubling is still happening.

Conclusion: I don't think it's just bigger interface files. I there is
some other space leak too.